The invention relates to a storage type smoke detector comprising a capacitor connected to a power supplying line and a discharge control circuit for periodically discharging a voltage charged in the capacitor, and particularly to a storage type smoke detector having a field effect transistor for operating to periodically close a discharge control circuit upon no detection of a fire and to open this circuit upon detection of a fire.
A known storage type smoke detector is disclosed in U.S. Pat. No. 3,872,449, which is based on a time difference between a long time constant to integrate smoke detection output and a short time constant to discharge as shown in the FIG. 1. A charging path comprising a series resistor R and capacitor C is connected to an output end of a threshold detector and a discharging path connected to the charging path is conprised of a series diode and resistor r. The capacitor C is charged by a fire indication signal exceeding the threshold level and is rapidly discahrged upon drop in signal level exceeding the threshold level. Since the threshold detector is normally in a OFF state for so long time before a fire arises, it is liable to conduct wrongly by a disturbance noise intruded from a power supplying line or induced by a high frequency electromagnetic wave. Such disturbance noise occasionally reaches 500-700 volts.
Another storage type smoke detector is comprised of a capacitor connected in parallel to a discharging circuit having a pair of transistors. The transistor pair which periodically switches on to close the discharging circuit is further coupled with and controlled by a third transistor which may conduct upon receipt of a smoke detection output from a smoke sensor. One transistor of the transistor pair defines a path to discharge a capacitor voltage while the other transistor holds the former conductive, and this transistor pair is adapted to operate with pulse signals. Since the third transistor is normally in a OFF state until a fire arises, it is also liable to wrongly conduct upon receipt of such disturbance noise so as to continuously charge the capacitor without discharge.
A general smoke detector has a very high impedance in order to reduce a power current dissipation for a long time before a fire arises, and accordingly, such a transistor that is normally put in a OFF state until it turns on to stop switching the transistor pair thereby continuing to charge the capacitor upon arising of a fire, is liable to wrongly turn on and to continuously charge the capacitor by disturbance noise, inspite of no fire. However, because a high impedance circuit is likely to respond to a disturbance noise to drop the impedance, a transistor for controlling a discharging circuit of the capacitor should be normally conductive thereby to render less susceptible to disturbance noises.